Fluid dynamic bearing lubricant air extraction device

ABSTRACT

The fluid dynamic bearings system of the invention provides a device located in a non-grooved low pressure region located at some point in the overall fluid circulation path of the fluid dynamic bearing to collect and trap any air bubbles which may be found in the fluid of the hydrodynamic bearing. More specifically, the air extraction device of the invention comprises a shallow angle V-shaped region which is located in fluid communication with but not in a grooved region of the hydrodynamic bearing. In summary, according to the present invention the hydrodynamic bearing comprises a shaft with a thrust plate at or near at least one end thereof. The thrust bearings are formed on the upper and lower surfaces of the thrust plate ( 301 ) and journal bearing ( 320 ) on the shaft or facing sleeve surface. Lubricant lies between each of these surfaces and facing surface of a sleeve ( 312 ) or counter-plate ( 308 ) which overlies the thrust bearing, and fluid lies in all these regions. In the region on the outer surface of the counter-plate distant from the shaft ( 322 ) and facing the counter-plate ( 308 ) a countersink ( 340 ) is formed on the axial face of the thrust plate ( 301 ) and end of bearing shaft with a shallow angle such that the force of surface tension forms a meniscus between the air and the lubricant along the surface of the countersink angle period. Lubricant circulation path-holes ( 380,382 ) are provided from, this countersink region to the region at the junction between the shaft and the thrust plate which also between the journal bearing and the thrust bearing. Thus the fluid can pass through these circulation holes extending from the journal and thrust plate up to the countersink region, and form a capillary seal on either side of the countersink region adjacent the thrust bearing. As an alternative, the shallow angled region may also be located on the outer diameter of the thrust plate or on the shaft adjacent the journal bearing.

[0001] This application claims benefit of U.S. Provisional ApplicationNo. 60/236,008, filed Sep. 27, 2000 entitled FLUID DYNAMIC BEARINGLUBRICANT AIR EXTRACTION DEVICE all of which is hereby incorporated byreference in its entirety.

FIELD OF THE INVENTION

[0002] The invention relates to the field of hydrodynamic bearingassemblies. More specifically, the invention relates to the design thatreduces the effect of air in the fluid of a hydrodynamic bearing.

BACKGROUND OF THE INVENTION

[0003] Disc drive memory systems have been used in computers for manyyears for storage of digital information. Information is recorded onconcentric memory tracks of a magnetic disc medium, the actualinformation being stored in the form of magnetic transitions within themedium. The discs themselves are rotatably mounted on a spindle. Theinformation is accessed by means of read/write heads generally locatedon a pivoting arm that moves radially over the surface of the disc. Theread/write heads or transducers must be accurately aligned with thestorage tracks on the disc to ensure proper reading and writing ofinformation.

[0004] During operation, the discs are rotated at very high speedswithin an enclosed housing by means of an electric motor generallylocated inside a hub that supports the discs. One type of motor incommon use is known as an in-hub or in-spindle motor. Such in-spindlemotors typically have a spindle mounted by means of two ball orhydrodynamic bearing systems to a motor shaft disposed in the center ofthe hub. Generally, such motors include a stator comprising a pluralityof teeth arranged in a circle. Each of the teeth support a plurality ofcoils or windings that may be sequentially energized to polarize thestator. A plurality of permanent magnets are disposed in alternatingpolarity adjacent the stators. As the coils disposed on the stators aresequentially energized in alternating polarity, the magnetic attractionand repulsion of each stator to the adjacent magnets cause the spindleto rotate, thereby rotating the disc and passing the information storagetracks beneath the head.

[0005] The use of hydrodynamic bearing assemblies in such drive systemshas become preferred due to desirable reductions in drive size and noisegeneration as compared to conventional ball bearing drive systems. Inhydrodynamic bearings, a lubricating fluid, such as oil or air,functions as the bearing surface between a base or housing and a spindleor hub. As the lubricating fluids require small gaps between thestationary and rotating members in order to provide the support,stiffness and lubricity required for proper bearing operation,conventional drive components and assemblies typically require tighttolerances and demand precision assembly methods.

[0006] Despite the use of such precise and controlled assembly methods,air bubbles may be introduced into the fluid which supports therelatively rotating services for rotation of the bearing assembly. Thusthe problem presented is to establish a reliable bearing design in whichthe possibility of the existence of air bubbles in the fluid between therelatively rotating grooved hydrodynamic bearing surfaces is diminished.

[0007] More specifically, in fluid dynamic bearings, a important goal islow non-repeatable runout (NRR) to optimize tracking and track density.In a fluid dynamic bearing motor, one potential source of NRR is thepresence of air in the grooved regions of the bearing, causing lubricantpressure instability and consequential rotor displacement. The presenceof air in the bearing lubricant can result from partial fill of thebearing cavity with lubricant or air ingestion due to a combination ofconditions including thermal contraction of the lubricant and parttolerances such as cylindrical taper in a journal bearing orsymmetrically formed bearing grooves. Due to the lubricant's tendency toflow throughout the bearing due to pressure gradients caused by parttolerances, air bubbles can be swept into the grooved regions of thebearing, resulting in NRR events. Therefore, the problem presented is toadopt a design which eliminates or diminishes the problem of air bubblesbeing swept into or residing in the grooved bearing regions of ahydrodynamic bearing.

SUMMARY OF THE INVENTION

[0008] The present invention seeks to provide a method and apparatus forminimizing or diminishing the tendency of air bubbles to be swept intogrooved regions of a hydrodynamic bearing.

[0009] More specifically, the present invention seeks to improve theoperation of a hydrodynamic bearing by providing a solution to theproblem created by air ingestion into a fluid in a hydrodynamic bearingof unstable operation.

[0010] Yet another objective of this invention is to provide a novelmethod and apparatus for collecting and trapping air bubbles as theycirculate through a fluid dynamic bearing.

[0011] A related objective of the invention is to provide a novel systemand method for trapping air bubbles in a hydrodynamic bearing byestablishing a device to capture air bubbles that is located in a regionof low pressure which is in the fluid path of the hydrodynamic bearingbut outside of the grooved portion of the hydrodynamic bearing. Bykeeping the air bubbles outside of the grooved region of the bearing,this air extraction device protects the grooved regions of the bearingfrom pressure gradient disruptions and resulting NRR events.

[0012] In summary, according to the present invention, the fluid dynamicbearings system of the invention provides a surface tension seal in anon-grooved low pressure region located at some point in the overallfluid circulation path of the fluid dynamic bearing to collect and trapany air bubbles which may be found in the fluid of the hydrodynamicbearing.

[0013] More specifically, the air extraction device of the inventioncomprises a shallow V-shaped feature or counter sink which is located influid communication with but not in a grooved region of the hydrodynamicbearing.

[0014] In summary, according to the present invention the hydrodynamicbearing comprises a shaft with a thrust plate at or near at least oneend thereof. Thrust bearings are formed on the upper and lower surfacesof the thrust plate and a journal bearing on the shaft or facing sleevesurface. Fluid is maintained between the grooved surface of a sleeve orthrust plate and the facing surfaces.

[0015] According to the invention, in the region on the outer surface ofthe thrust plate distant from the shaft and facing the counterplate, ashallow angle countersink is defined in the low pressure region adjacentto but separate from the thrust bearings. The shallow angle of thecountersink is such that the force of surface tension forms a meniscusbetween the countersink surface and the facing counterplate surfacewhich separates the air and the lubricant.

[0016] In the preferred embodiment, lubricant re-circulation path-holesare provided from this countersink region to a region at a junctionbetween the, shaft and the thrust plate i.e. between the journal bearingand the thrust bearing.

[0017] Thus the fluid can pass through these re-circulation pathsextending from the journal and thrust plate up to the countersinkregion, a capillary seal is formed surrounding the countersink regionadjacent the thrust bearing. The shallow angles of the countersinkregion utilize surface tension to form a capillary seal just inside thecountersink region adjacent the thrust bearing. This capillary sealdefines the low while allowing any air bubbles to be attracted to thisregion and then trapped within this region, extracting the air bubblesfrom the lubricant.

[0018] In an alternative approach, a shallow angled region may be formedat the radial end of the thrust plate in the fluid circulation patharound the thrust plate so that any air bubbles will tend to be drawnfrom the fluid into this low pressure region. In yet another alternateembodiment, the shallow angled region is defined adjacent the shaft inthe low pressure region between the journal bearings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] So that the manner in which the above recited features of thepresent invention are obtained and can be understood in detail, a moreparticular description of the invention, briefly described above, may behad by reference to embodiments which are illustrated in the followingdrawings. However, it is to be noted that the following drawingsillustrate only typical embodiments of the invention and are thereforenot to be considered limiting of its scope, for the invention may admitto other equally effective embodiments.

[0020]FIG. 1 is a top plan view of a disc drive data storage device inwhich the present invention may be especially useful;

[0021]FIG. 2 is a vertical sectional view of a typical disc drivespindle motor in which the present invention may prove to be useful;

[0022]FIGS. 3A and 3B are partial sectional view of the shaft thrustplate and counterplate of a hydrodynamic bearing showing the countersinkand re-circulation paths are utilized to trap and eliminate air bubblesin the present invention.

[0023]FIGS. 4A and 4B illustrate further alternative embodiments of, theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] The invention comprises a spindle motor for a disc drive datastorage device having a thrust plate type hydrodynamic bearing. FIG. 1is a plan view of a typical disc drive 10 wherein the invention isuseful. Disc drive 10 includes a housing base 12 and a top cover 14. Thehousing base 12 is combined with top cover 14 to form a sealedenvironment to protect the internal components from contamination byelements from outside the sealed environment.

[0025] The base and top cover arrangement shown in FIG. 1 is common inthe industry. However, other arrangements of the housing components havebeen frequently used, and the invention is not limited to theconfiguration of the disc drive housing. For example, disc drives havebeen manufactured using a vertical split between two housing members. Insuch drives, that portion of the housing half that connects to the lowerend of the spindle motor is analogous to base 12, while the oppositeside of the same housing member, that is connected to or adjacent thetop of the spindle motor, is functionally the same as the top cover 14.

[0026] Disc drive 10 further includes a disc pack 16 that is mounted forrotation on a spindle motor (not shown) by a disc clamp 18. Disc pack 16includes one or more individual discs that are mounted for co-rotationabout a central axis. Each disc surface has an associated head 20 thatis mounted to disc drive 10 for communicating with the disc surface. Inthe example shown in FIG. 1, heads 20 are supported by flexures 22 thatare in turn attached to head mounting arms 24 of an actuator body 26.The actuator shown in FIG. 1 is of the type known as a rotary movingcoil actuator and includes a voice coil motor (VCM), shown generally at28. Voice coil motor 28 rotates actuator body 26 with its attached heads20 about a pivot shaft 30 to position heads 20 over a desired data trackalong an arcuate path 32. While a rotary actuator is shown in FIG. 1,the invention is also useful in disc drives having other types ofactuators, such as linear actuators.

[0027]FIG. 2 shows a rotating shaft 100 spindle motor design in whichthe shaft is integrated with the hub 102 which carries flange 103 whichfunctions as a disc support surface. The shaft with the hub 102 supportsa magnet 104 on its inner axial surface, facing stator 106 whoseenergization causes stable rotation of the hub. The stator in turn issupported on an axial extension 108 of base casting 110. A sleeve 112which supports the shaft 100 and its associated thrust plate 116 isincorporated into the axial extension 108 of the base 110. This sleeve112 has axial surface 120 that faces a surface of the shaft. These twosurfaces define a journal bearing which is of standard design and notfurther shown. Further, the thrust plate at surfaces 122 and 124 definein cooperation with the sleeve 112 and the counterplate 130 thrustbearings of the fluid dynamic type which further support the shaftagainst axial forces. Each of these journal and thrust bearings requirefluid in the gap between the facing surfaces. This fluid may eitherrecirculate through an internal channel 134 which either passes throughthe thrust plate or between the thrust plate and shaft, or through acentral bore. To prevent the escape of any fluid between the surface 140of the sleeve and the complementary surface 142 of the thrust plate, alaser weld has been applied at the junction at the axially outer edge ofthe counterplate 130 and the sleeve 112. This laser weld is appliedusing well-known techniques and technology but by its very simplicityenhances the reliability.

[0028] Given the knowledge of the basic operation of a thrust plate andshaft combination to support rotation of the hub, attention is directednext to FIGS. 3A and 3B. FIG. 3A shows the combination of thrust plateand shaft without the lubricant needed to support relative rotation ofthe bearing parts; FIG. 3B shows the system with the lubricant.Referring first to FIG. 3A, this figure shows thrust bearings 302, 304on the upper and lower surfaces of the thrust plate 301, facingrespectively a surface 306 of counterplate 308 and a surface 310 ofsleeve 312. The same figure also shows a journal bearing 320 defined onthe outer surface of the shaft 322, and cooperating with the innersurface 324 of the sleeve 312 to form a journal bearing. The thrustbearings and journal bearings together support the shaft 322 and sleeve312 for relative rotation. In this figure, the grooves which define thebearing appear on the surface of the thrust plate and shaft; however,they can just as well be defined on the corresponding surface of thecounterplate 308 and sleeve 312.

[0029] It is well-known and has been described above, that it isessential to avoid nonrepeatable runout and maintain proper anduninterrupted support for the relative rotation of shaft, thrust plateand sleeve and thereby rotation of the disc or discs supported on thesleeve, that no air bubbles occur in the lubricant which provides thebearing. Despite the best defined approaches to assembling and fillingthe hydrodynamic bearing, it is not uncommon for bubbles to occasionallyto be found in the system of a hydrodynamic bearing. Therefore,according to the present invention, the design shown in FIG. 2 ismodified. In a first embodiment of the invention, a shallow anglecountersink 340 is defined on the axial face 342 of the end of thebearing shaft or the thrust plate surface. The countersink is formedwith a sufficiently shallow angle 350 that the force of the surfacetension between the two angled surfaces which are the end surface orface surface of the thrust plate 342 and the facing surface 306 of thecounterplate that a meniscus 352 (see FIG. 3B) is defined between theair 360 and the lubricant 362. This design modification establishesmeans for entrapping air bubbles formed in the fluid circulation path inthe low pressure region adjacent the thrust bearing on the axial surfaceof the thrust plate. The shallow angle 350 of the countersink surfacerelative to the facing surface 306 of the counterplate utilizes surfacetension to form the capillary seal or surface tension meniscus 352,which will retain the large air bubble 360 in this low pressure regionwherein air bubbles will congregate and be trapped.

[0030] To further optimize benefits of this countersink, a furthermodification of the design of FIG. 2 is proposed. Specifically, withoutany further modification, the circulation holes 134 (FIG. 2) which aredefined between the thrust plate and the outer surface of the shaftwould terminate facing the counterplate either close adjacent to thecapillary seal or even outside of the capillary seal. Thus, it would bepossible for fluid to circulate through the system without being passedthrough the counter sink region and the countersink 340 formed in thecountersink region, thereby not achieving the full benefits of thisinvention.

[0031] Therefore, the recirculation path holes which extend from theregion intermediate the thrust plate bearing 304 and the journal bearing320 are redirected through the shaft to lie at a fairly shallow angleand run preferably to the shallowest portion, but at least to some partof the region inside the meniscus 352. This connection of therecirculation path 380, 382 which may comprise one or a plurality ofholes running at a fairly shallow angle from, as shown in thisembodiment (and preferably, but not necessarily at this corner) a regionbetween the thrust plate bearing and the journal bearing corner formedby the thrust plate and shaft, to a point in the countersink 340. It hasbeen found that with typical fluid circulation caused by the pumpingeffect of the thrust bearing grooves and journal bearing grooves, thatbubbles to the extent that they exist, will tend to move into theserecirculation path holes 380,382 and the circulation will then tend tocause these bubbles to move into the countersink 340 and be retainedthere by this meniscus 352. Once these bubbles are drawn into the lowpressure region in the countersink, the air bubbles are trapped by thecapillary seal or surface tension meniscus 352.

[0032] Among the many advantages of adoption of this design in additionto the apparent one of implementing trapping of the air bubbles in thesystem, is that this modified design is easy to manufacture, inrequiring only the shallow countersink at the end of the shaft or at thesurface of the thrust plate aligned with the end of the shaft; and theprovision of both plurality of the easily formed shallow holes 380, 382extending from the region between the journal bearing and thrust bearingto a point inside the countersink 340. Addition of this countersink 340at a shallow angle relative to the surface thrust plate in which thecounter sink is defined causes the low pressure region to add littlelubricant volume to the overall fluid dynamic bearing, and therefore haslittle impact on the lifetime of the bearing.

[0033] Further, the connection of the countersink to the recirculationpath by the modified lubrication recirculation path holes 380,382incorporates these recirculation path holes into the air bubblecollection process, and optimizes the air bubble collection process.

[0034] Two further alternative embodiments appear in FIGS. 4A and 4B.Both of these comprise alternative modifications to the motor andbearing design shown, for example, in FIG. 2, although these designscould be used to enhance the operation of any combination of a shaftsupporting a thrust plate for relative rotation with a surroundingsleeve and counterplate. In the embodiment of FIG. 4A, a shaft 410 isshown supporting a thrust plate 412 at an end thereof. A sleeve 414supports a counterplate 416 so that relative rotation between the sleeveand the shaft can be supported by the fluid in the gap 418. As iswell-known in this field and described above with respect to theembodiments of FIGS. 3A and 3B, thrust bearings 420, 422 are providedbetween the axial surfaces of the counterplate and the facing surfacesof the sleeve 414 and the counterplate 416. Fluid circulates as shown inthe figure, both over the outer surfaces of the shaft 418, over theradial surfaces 430, 432 of the thrust plate, over the outer diametersurface 434 of the thrust plate, and through the circulation holes oropenings 440, which connect the gaps adjacent the axial surfaces of thecounterplate 440 and the gap adjacent the shaft 418 where journalbearings are typically defined. In order to provide the means forentrapping air bubbles in this fluid circulation path, shallow anglev-shaped region 450 is defined in the outer diameter surface 452 of thethrust plate 412.

[0035] As explained above, with respect to the embodiment of FIGS. 3Aand 3B, this shallow angle region causes the formation of a meniscus ateither end thereof, creating a fairly large air bubble in the shallowangle region in which air bubbles which would normally circulate throughthe circulation path and pass over the grooved regions of the thrustbearing and/or journal bearings entrapped in this major air bubble 450.Therefore, by adopting this design, stability of the system is enhanced.

[0036] In yet another alternative approach, shown in FIG. 4B, across acombination of shaft 410 and thrust plate 412 is again provided forestablishing relative rotation between that combination and the sleeve414 and counterplate 416. In this design, the return path is definedbetween the journal bearings 425, 427 which support the shaft 410 forrotation, and extends radially away from the low pressure region 460between these journal bearings towards the center of the shaft. The paththen extends axially through the center of the shaft 462 to exit at thecenter of the thrust bearing 464 on the axially outer surface of thethrust plate 412. In this design of FIG. 4B, the shallow angle v-shapedregion 470 is defined at this low pressure area 472 between the journalbearings. As with the previous designs, by establishing this shallowangle v-shaped region, a meniscus 476 is formed around the low pressureregion defining a major air bubble 472 therein. This air bubble lies inthe low pressure region between the journal bearings 425 and 427, and inthe fluid circulation path which includes the gap between the thrustplate and the shaft and surrounding sleeve and counterplate, and thereturn path through axial bore 462 and radial bore 461. Therefore, anybubbles which are in this path will tend to become entrapped in thelarge air bubble in the shallow v-shaped region 460, and taken out ofcirculation in the system.

[0037] Other features and advantages of this invention as well asalternative approaches to defining the shallow v-shaped region in a lowpressure area of the fluid circulation path in a hydrodynamic bearingdesign will become apparent to a person of skill in the art who studiesthis disclosure. Therefore, the scope of this invention is to be limitedonly by the following claims.

What is claimed is:
 1. A hydrodynamic bearing comprising a shaft with athrust plate mounted at an end thereof, a sleeve mounted for rotationrelative to the shaft, a counterplate supported from the sleeve andoverlying the thrust plate, the shaft having a journal bearing definedbetween the shaft outer surface and an inner surface of the sleeve, athrust bearing on a surface of the thrust plate between the thrust plateand the sleeve, each of the journal bearing and thrust bearing beingestablished by fluid maintained and circulated in a gap between theshaft and thrust plate and the sleeve, the fluid being maintained in agap which extends between the shaft and the sleeve and between axiallyupper and lower and radially outer surfaces of the thrust plate and thesurrounding sleeve and counterplate and between the shaft andsurrounding sleeve and a shallow angle v-shaped region defined in a lowpressure region in the gap.
 2. A hydrodynamic bearing as claimed inclaim 1 wherein an axially upper surface of the thrust plate includes athrust bearing defined thereon facing the counterplate, and wherein theshallow V-shaped region is in the region which is radially interior tothe thrust bearing.
 3. A hydrodynamic bearing as claimed in claim 2wherein the shallow region is a substantially V-shaped groove, with thecenter of the V being at the center axis of the shaft.
 4. A hydrodynamicbearing as claimed in claim 2 wherein the shaft extends through thethrust plate to face the counterplate, and wherein the shallow V-shapedcountersink is defined at the end of the shaft.
 5. A hydrodynamicbearing as claimed in claim 4 wherein the shallow V-shaped region is acountersink coaxial with the center axis of the shaft.
 6. A fluiddynamic bearing as claimed in claim 4 wherein the countersink is at anangle of about 8-12 degrees with the surface of the thrust plate.
 7. Ahydrodynamic bearing as claimed in claim 3 including one or more fluidcirculation holes extending from a point between the journal bearing andthe thrust bearing on the axially lower surface of the thrust plate at ashallow angle to intersect the countersink defined on the upper surfaceof the thrust plate.
 8. A hydrodynamic bearing as claimed in claim 7wherein each of the shallow fluid circulation holes meet at about thesame point at the center axis of the shaft and countersink.
 9. Ahydrodynamic bearing as claimed in claim 8 wherein the fluid circulationholes meet the axially upper surface of the thrust plate at a pointwhich is radially interior to the thrust bearing.
 10. A hydrodynamicbearing as claimed in claim 9 wherein the countersink is at an angle ofabout 8-12 degrees with the surface of the thrust plate.
 11. Ahydrodynamic bearing as claimed in claim 1 wherein the shallow angleregion is defined in the gap at the radial outer diameter of the thrustplate.
 12. A hydrodynamic bearing as claimed in claim 11 wherein theshallow angle recess occupies substantially the entire outer diametersurface of the thrust plate.
 13. A hydrodynamic bearing as claimed inclaim 1 wherein the shaft supports the shallow angle region in thesurface of the shaft adjacent the journal bearing.
 14. A hydrodynamicbearing as claimed in claim 13 wherein the shallow angle region occupiesthe low pressure region intermediate first and second journal bearingsdefined along the shaft to support the shaft for rotation and the sleevefor relative rotation.
 15. A hydrodynamic bearing as claimed in claim 14wherein the shaft includes a bore extending radially from the lowpressure region to about the center of the shaft, and axially to an exitat the distal surface of the thrust plate intermediate the thrustbearing.
 16. A spindle motor for use in a disc drive comprising a shaftand a sleeve mounted for relative rotation, the sleeve supporting a hubsupporting one or more discs thereon for constant speed rotation asdriven by said motor, and the shaft having a journal bearing definedbetween the shaft outer surface and an inner surface of the sleeve, athrust bearing on a surface of the thrust plate between the thrust plateand the sleeve, each of the journal bearing and thrust bearing beingestablished by fluid maintained and circulated in a gap between theshaft and thrust plate and the sleeve, the fluid being maintained in agap which extends between the shaft and the sleeve and between axiallyupper and lower and radially outer surfaces of the thrust plate and thesurrounding sleeve and counterplate and between the shaft andsurrounding sleeve and a shallow angle v-shaped region defined in a lowpressure region in the gap.
 17. A spindle motor as claimed in claim 1wherein the shallow angle region is a countersink region in an end ofthe shaft extending through the thrust plate.
 18. A spindle motor asclaimed in claim 1 wherein the shallow angle region is defined in a lowpressure region intermediate thrust bearings on axially upper and lowersurfaces of the thrust plate, and is defined on a radially outerdiameter of the thrust plate.
 19. A spindle motor as claimed in claim 1wherein the shallow angle region is defined in a low pressure regionseparate from the journal bearing supporting the shaft and thrust platefor relative rotation.
 20. A hydrodynamic bearing comprising a shaftwith a thrust plate mounted at an end thereof, a sleeve mounted forrotation relative to the shaft, a counterplate supported from the sleeveand overlying the thrust plate, the shaft having a journal bearingdefined between the shaft outer surface and an inner surface of thesleeve, a thrust bearing on a surface of the thrust plate between thethrust plate and the sleeve, each of the journal bearing and thrustbearing being established by fluid maintained and circulated in a gapbetween the shaft and thrust plate and the sleeve, the fluid beingmaintained in a gap which extends between the shaft and the sleeve andbetween axially upper and lower and radially outer surfaces of thethrust plate and the surrounding sleeve and counterplate and means forentrapping air bubbles in the fluid in said gap in a low pressure regionspaced from the fluid bearings.